The invention relates generally to acrylate/methacrylate adhesives, including those used to bond metal substrates and metal objects. More particularly, the invention relates to such adhesives having improved storage stability, improved toughening and impact properties, and reduced weight loss during cure. While the present adhesives may be directed to bonding metal components, it should be emphasized that the present adhesives may also be used to bond non-metal components.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Acrylate and methacrylate-based adhesives are used to bond components, such as in the construction and repair of cars, trucks, trailers, buses, boats, and other products and structures. Typically, two parts of the adhesive system are prepared and stored prior to mixing the two parts to give the final adhesive. In certain formulations, one part of the adhesive may be labeled the adhesive part or adhesive side, and the other part may be labeled the activator part or activator side. Typically, the adhesive part includes the acrylate and methacrylate monomers to be polymerized, and a reducing agent. The activator part typically includes initiators of the polymerization, such as peroxides.
During curing of the mixed adhesive parts, the reaction or polymerization of the acrylate and/or methacrylate monomers is exothermic. Thus, the adhesive typically experiences an increase in temperature until a peak exotherm temperature is reached. After the adhesive reaches its peak exotherm temperature, the temperature of the adhesive may gradually return to ambient temperature. On the other hand, external heat may be applied to the curing adhesive prior to allowing the adhesive to return to ambient temperature. Cure characteristics or the cure profile of an adhesive may include the peak exotherm temperature, the time to reach the peak exotherm temperature, the adhesive working time (or open time), the adhesive fixture time, and so forth.
A problem with acrylate/methacrylate-based structural adhesives, such as those used to bond metal components, is poor storage stability of the adhesive part and/or activator part. Storage instability can cause stringiness (i.e., formation of conglomerations or strings of the adhesive), an increase in viscosity, reduced mixability of the adhesive, and so forth. Unfortunately, such undesirable properties caused by poor storage stability may result in inadequate wetting of the adhesive, insufficient or inconsistent adhesion, and other problems.
Another common problem with acrylate/methacrylate-based adhesives is low impact strength of the cured adhesives at low temperatures, e.g., −40° C. (−40° F.). While the cured adhesives generally have desirable properties (e.g., lap shear strength) at elevated temperatures, such as 82° C. (180° F.), the performance (e.g., impact strength) of the cured adhesives at low temperatures is generally a weakness for these types of adhesives. Unfortunately, toughening techniques that improve the low-temperature performance of the cured adhesive may sacrifice high temperature performance of the cured adhesive significantly. In general, an adequate combination of adhesive properties at these high and low temperatures may be important because customers and users of the adhesives may specify requirements of the adhesive properties at relatively extreme temperatures falling outside of typical ambient ranges, i.e., as a design margin or testing standard, for example. Moreover, an adequate combination of adhesive properties at high and low temperatures are generally important for products having parts bonded with acrylate/methacrylate-based structural adhesives that may be subjected to various weights and forces in a wide range of environments and ambient temperatures.
Lastly, acrylate/methylcrylate-based structural adhesives, including those used to bond metal components and substrates, may experience excessive weight loss during curing of the adhesives. Generally, weight loss and the associated offensive odors result from volatization of the adhesive ingredients such as the monomer. Weight loss of the adhesive during cure is believed to be exacerbated by use of lower molecular-weight acrylate/methacrylate monomers having low boiling points and low flash points, as well as by the exothermic polymerization (or reaction) and high exotherm temperatures, and so on.
There is a need with acrylate/methacrylate-based structural adhesives, including those used to bond metal components and metal substrates, to improve storage stability of the adhesive/activator parts. There is also a need to improve low-temperature performance (e.g., impact strength) of the cured adhesives without significantly sacrificing high-temperature performance. Moreover, there is a need to reduce weight loss of the adhesive during cure.